1. Field of the Invention
The present invention relates to a direct access storage device (DASD) of the type that includes a motor, and more particularly to a method and apparatus for controlling a motor within a DASD using pulse width modulated control signals.
2. Description of the Related Art
Direct access storage devices (DASDs) incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the surfaces of the disks. Typically, the disks are mounted in parallel for simultaneous rotation and by an integrated spindle and motor assembly. Transducer heads are driven in a path toward and away from a drive axis in order to position the heads over the disks to allow data to be written to the disks and read from the disks. Data is recorded in data information tracks provided on the surfaces of the disks.
FIG. 1 is a block diagram of a prior art DASD 100 having a spindle motor and a voice coil motor 128. The spindle motor 102 causes a disk 104 to rotate about a spindle 106. A voice coil motor (VCM) 128 causes a read/write head 108 to be positioned over the disk 104. The disk 104 is treated with magnetic material which allows information to be encoded thereon by manipulation of a magnetic field generated by the read/write head 108. Information that is read and written to the head is coupled to and from the read/write head 108 through a read/write preamplifier 110.
Additional data (commonly referred to as "servo" information) is stored with the user data on the disk. Servo information provides feedback for the VCM regarding the current position of the read/write head 108 to allow the read/write head to be properly positioned over known locations on the disk at which data is to be stored, and from which data is to be read. A servo demodulator 112 distinguishes the servo information from user data. User data is coupled through a data channel 114 to an interface microprocessor 116 which controls the communication of the user data to and from a host computer (not shown) through an interface controller 118. The servo data is coupled to a servo digital signal processor (DSP) 120.
The servo DSP 120 processes the servo information in order to produce control signals which control the speed of the spindle motor 102 and the motion of the VCM. The control signals that are generated by the servo DSP 120 are coupled to a VCM/spindle pre-driver circuit 122. The VCM/spindle pre-drive circuit 122 generates drive signals to a VCM power amplifier 124 and to a spindle power amplifier 126. The VCM power amplifier 124 in turn drives the VCM 128 to position the read/write head appropriately. Likewise the spindle power amplifier 126 drives the spindle motor 102 to cause the spindle motor 102 to achieve and maintain a desired rotational velocity.
Typically, a brushless, direct current (DC) spindle motor is used in disk drives. Spindle motor control is accomplished in accordance with one of two methods; a linear drive method or a pulse width modulation (PWM) drive method. Similarly, the VCM can be driven by either a linear drive method or a PWM drive method. Linear drive requires that current be continuously applied to the windings of the motor. In contrast, a PWM drive method requires that current is provided in pulses to the windings. A hybrid method can be used for motor control by selecting PWM drive during one portion of the operation of the DASD, and linear drive during other modes of operation. The PWM drive method has been used to improve efficiency and thus limit the high-power dissipation, particularly in the spindle motor at startup. Because current is pulsed through the windings of the spindle motor, the overall power required to rotate the motor in accordance with PWM drive techniques is less than that required for driving the motor in accordance with the linear drive method. However, pulsing the current through the winding and drive circuit components generates electrical noise which can be coupled to other circuits in the DASD. In particular, the relatively fast rise and fall times that occur in the transitions between on and off times tend to generate harmonic overtones which result in high frequency electrical noise. Accordingly, the linear drive method is the more popular than PWM drive because less electrical noise is generated.
One commercially available drive device, part number L6232, is manufactured and sold by SGS-Thompson Microelectronics for use in brushless DC motor applications, and particularly for use with disk drive applications. This device includes circuitry to perform both PWM drive and linear drive. The spindle driver is described for use to limit the high-powered dissipation that occurs during spindle startup by employing PWM drive during start-up, and then switching to linear drive when the disk approaches normal operating spindle speed, thus reducing the noise generated by the PWM drive. However, by switching to linear drive as soon as the disk reaches (or approaches) operating speed, the benefits of PWM drive are realized only during start-up. More particularly, the improved power efficiency that could be achieved by use of PWM drive cannot be used during at least some operations (such as reading and writing) due to the electrical noise that is typically generated by the PWM drive method.
Therefore, while the prior art motor control systems provide generally effective operation, it is desirable to provide an improved method and apparatus for spindle mode control in order to provide reduced power consumption by use of PWM drive, while reducing the electrical noise generated by the PWM drive method in order to limit disruption of the operation of the DASD.
Furthermore, in a typical three phase motor, three motor windings are provided through which current flows in a sequence known as a "commutation" scheme. The commutation scheme determines when current is to flow through each of the windings. FIG. 2 is an plot of the current that flows through the three windings of a three phase motor in accordance with one commutation scheme. FIG. 3 is a schematic of the output stages of a power amplifier 126 which drives the windings 301, 303, 305 of a three phase motor. It can be seen from FIG. 2, that at any one time, one winding is conducting current in a positive direction, one winding is conducting current in a negative direction, and one winding is not conducting at all. For the purpose of this discussion, it can be assumed that the positive direction of current flow is from the power amplifier toward the motor and that the negative direction of current flow is from the motor toward the power amplifier. From FIG. 3 it can be seen that in order to have current flow through any one winding, current must also flow through at least one other winding. That is, current flows from the positive power supply through one of three upper output transistors 307, 311, 315, then through the each of the windings coupled to each upper transistor that is conducting, assuming that at least one of the lower transistors 309, 313, 317 are also conducting, and through the winding that is coupled to the conducting lower transistor.
From FIG. 2 it can be seen that there are six distinct commutation phases, noted as phase 1 through phase 6 in the figure. Note that phase 1 and phase 2 are repeated in the figure. In each such phase, current flows through only two windings. For example, in the first phase, winding "A" is conducting in the positive direction, winding "B" is not conducting, and winding "C" is conducting in the negative direction. Accordingly, in phase 1 of the commutation scheme, only output transistor 307 coupled to winding A and output transistor 317 coupled to winding C are conducting. Each of the other four output transistors 309, 311, 313, 315 are "turned off" (i.e., not conducting).
In the second phase, output transistor 317 turns off and output transistor 313 turns on. As can be seen from FIG. 2, there is a perturbation in the current that flows through winding A due to the ramping down of the current in winding C (i.e., output transistor 317 turning off) before the current through winding B can achieve sufficient level to maintain the current in winding A (i.e., before output transistor 313 can start conducting sufficiently). This perturbation in the current through the windings of the motor causes a mechanical vibration which occurs at the commutation frequency in the motor, associated enclosure, and structures to which the motor enclosure is mechanically coupled. In many cases, the commutation frequency is in the audio range. For example, in one such spindle motor drive circuit, the commutations occur at 2.88 kHz. Therefore, it would be desirable to eliminate the perturbation in the current in order to reduce the acoustic noise generated by operation of the spindle motor.